The present invention relates to the casting of metals by a lost foam process and more particularly to an improved coating for application to the polystyrene pattern used in such lost foam process.
The lost foam process for casting of metals is described in U.S. Pat. Nos. 4,482,000 and 4,448,235, the disclosures of which are expressly incorporated herein by reference. In such lost foam process, a polystyrene pattern is embedded in a sand mold. Molten metal poured into the mold vaporizes the polystyrene and occupies the void created thereby, thereby concurrently consuming the polystyrene pattern and forming a casting. The lost foam process is used where particularly intricate castings are to be formed. The lost foam process also can reduce the rigging required by conventional sand casting (e.g., sprues, risers, gates, downcomers, etc.).
A refractory coating typically is applied to the consumable pattern to, inter alia, thermally insulate the metal to prevent premature hardening of the metal before the entire pattern has been replaced by the molten metal. Additionally, turbulence of the metal is lessened by the insulating coating because the coating initially retards escape of the pattern decomposition vapors. Thereafter, the pattern decomposition vapors are vented from the casting through the coating and into the sand that surrounds the pattern. Such escape of the vapors proceeds by virtue of the heat from the molten metal vaporizing the organic content of the pattern coating through which the pattern vapors escape into the surrounding sand.
The '235 patent discloses the use of a first coating for the pattern which is a thermally insulative, relative gas permeable, refractory layer. A second layer is applied over the first layer and it is a vaporizable, relatively gas impermeable, polymeric layer. The refractory layer is applied as a water-based slurry and the polymeric layer as a water-based emulsion.
The '000 patent discloses a single coating for the pattern which is predominantly refractory particulates and polymeric particles vaporizable at metal casting temperatures but having a relatively low permeability to pattern decomposition vapors. A combination of polyethylene particles, xanthan gum, and calcium lignosulfonate binder is preferred.
Lustrous carbon defects in, for example, iron castings are a problem that is not addressed by the foregoing art. Lustrous carbon defects are believed to be caused by entrapped polystyrene decomposition products and typically occur on the top of the cope surface of the casting as a wrinkled skin appearance. The present invention is addressed to providing a consumable pattern coating that exhibits the desirable characteristics recognized by the art while concomitantly providing a diminution in lustrous carbon defects in metal castings manufactured by the lost foam process.